Process for preparing nanostructured materials of controlled surface chemistry

ABSTRACT

A process to prepare stoichiometric-nanostructured materials comprising generating a plasma, forming an “active volume” through introduction of an oxidizing gas into the plasma, before the plasma is expanded into a field-free zone, either (1) in a region in close proximity to a zone of charge carrier generation, or (2) in a region of current conduction between field generating elements, including the surface of the field generation elements, and transferring energy from the plasma to a precursor material to form in the “active volume” at least one stoichiometric-nanostructured material and a vapor that may be condensed to form a stoichiometric-nanostructured material. The surface chemistry of the resulting nanostructured materials is substantially enhanced to yield dispersion stable materials with large zeta-potentials.

THE FIELD OF THE INVENTION

[0001] The present invention is concerned generally with makingnanostructured materials using plasma technologies. More particularly,the invention is concerned with a method of making a variety ofstoichiometric-nanostructured materials by forming a unique “activevolume” in a plasma through the introduction of an oxidizing gas. Thesurface chemistry of the resulting nanostructured material issubstantially enhanced to yield dispersion stable materials with largezeta-potentials.

BACKGROUND OF THE INVENTION

[0002] Methods of plasma formation are previously known in the art andmay be selected from a group of comprising radio-frequency fields,microwave discharges, free-burning electric arcs, transferred electricarcs, high-intensity lasers, capacitively coupled electro-thermaligniters, DC glow discharges, and DC cold cathode discharges.

[0003] Methods for transferring energy to a precursor material byexposing a precursor material to the energy of a plasma are previouslyknown in the art. Precursor material may be introduced into a plasma atany point. For example, a plasma may be created by a high intensityelectric arc and a precursor may be introduced at any point of the arccolumn. In U.S. Pat. No. 3,209,193, the precursor material is introducedinto the arc column of a free-burning plasma at the anode and U.S. Pat.No. 3,900,762 describes a working embodiment of the volumetricintroduction of precursor into a plasma arc.

[0004] The precursor material may also be a consumable electrode. Forexample, in U.S. Pat. Nos. 5,460,701 and 5,514,349, a transferredelectric arc between a cathode and a consumable anode is used togenerate precursors in an elongated ionized arc that extends beyond theconduction columns.

[0005] Prior art teaches that materials formed by plasma techniques mayhave unusual properties. But prior art does not teach the synthesis ofstoichiometric-nanostructured materials with controlled surfacechemistry.

[0006] Materials produced by the method of this patent have surfacechemistry characterized by a high aqueous dispersion stability, a lowrate of hydrolysis, and a large zeta-potential. Materials produced bythe method of this patent are stoichiometricly-nanostructured by the“active volume”. The “active volume” is in a plasma and is created byintroducing an oxidizing gas into the plasma, before the plasma isexpanded into a field-free zone, either (1) in a region in closeproximity to a zone of charge carrier generation, or (2) in a region ofcurrent conduction between field generating elements, including thesurface of the field generating elements. Energy is transferred from theplasma to a precursor material and at least one of astoichiometric-nanostructured material and a vapor that may be condensedto form a stoichiometric-nanostructured material are formed in the“active volume”. The “active volume” is the most reactive part of theplasma and material synthesized in the “active volume” arestoichiometric-nanostructures with unique surface chemistry.

[0007] Stoichiometric-nanostructures or stoichiometriclly-nanostructuredmaterials are defined as materials having controlled chemistry at thenanoscale. The chemistry of the nanostructured material may becontrolled to be of full or partial stoichiometry, in the chemicalsense, with respect to a reactant.

[0008] Prior art does not teach the introduction of oxidizing gas in aplasma to nanostructure materials to have unique surface chemistry.Instead prior art teaches away from the use of oxidizing gases in aplasma. For example U.S. Pat. No. 3,899,573 teaches the use of areducing gas in the plasma created by a free-burning arc. The use ofoxidizing plasma environments is conventionally discouraged because thematerials used to generate the plasma are aggressively corroded. Forexample U.S. Pat. No. 4,642,207 discloses the use of an oxidizingplasma. But this process cannot be practiced in a manufacturingenvironment because aggressive corrosion rapidly renders processequipment inoperable. This is often the case even under conditions whereshielding gas flows are used to protect specific process equipment asdisclosed in prior art. The present invention teaches that judiciousformation of an “active volume” enables the use of an oxidizingenvironment within the conduction column of a variety of plasmas tosynthesize stoichiometric-nanostructured materials with unique surfacechemistry.

[0009] Prior art does not teach the importance of forming at least oneof stoichiometric-nanostructured material or vapor that may be condensedto form stoichiometric-nanostructured material in the “active volume” ofa plasma. Instead prior art transfers energy from a plasma to precursorsand forms nanoparticles by injecting at least one of a quench and areaction gas:

[0010] after the plasma is expanded into a field-free zone; and/or

[0011] down stream from either (1) a zone of charge carrier generation,or (2) a region of current conduction between field generating elements.

[0012] U.S. Pat. Nos. 5,460,701 and 5,514,349, use a transferredelectric arc between a cathode and a consumable anode to generate anelongated ionized arc that extends beyond the conduction columns andinjects at least one of a quench and a reaction gas into the elongatedionized arc. Other forms of the art introduce a reactive gas down streamfrom the “active volume” and form materials during thermal quench or gasphase nucleation. In all cases the art teaches the formation ofmaterials in less reactive plasmas.

[0013] Experiments in our laboratory indicate the “active volume” mustbe carefully controlled, to form before the plasma is expanded into afield-free zone, either (1) in a region in close proximity to a zone ofcharge carrier generation, or (2) in a region of current conductionbetween field generating elements, including the surface of the fieldgenerating elements, to derive the benefits of the reactive plasma andsynthesize a stoichiometricly-nanostructured material with uniquesurface chemistry.

OBJECTS OF THE INVENTION

[0014] An object of the present invention is the development of aprocess for producing stoichiometric-nanostructured materials. Thisprocess comprises the steps of:

[0015] generating a plasma;

[0016] forming an “active volume” through introduction of an oxidizinggas into the plasma, before the plasma is expanded into a field-freezone, either (1) in a region in close proximity to a zone of chargecarrier generation, or (2) in a region of current conduction betweenfield generating elements, including the surface of the field generatingelements; and

[0017] transferring energy from the plasma to a precursor material ormaterials and forming in the “active volume” at least one ofnanoparticles and a vapor that may be condensed to form a nanoparticle.

[0018] A further object of the present invention is the production ofstoichiometric-nanostructured materials with unique surface chemistrycharacterized by high aqueous dispersion stability, a low rate ofhydrolysis, and a large zeta-potential.

[0019] These and other objects of the invention will become moreapparent as the description thereof proceeds.

DESCRIPTION OF THE INVENTION

[0020] A free-burning electric arc is struck between anode and cathodeusing methods taught in U.S. Pat. Nos. 3,900,762, 3,899,573, and4,080,550. Plasma generation is not limited to free-burning arcs, butmay be selected from a group comprising radio-frequency fields,microwave discharges, free-burning electric arcs, transferred electricarcs, high-intensity lasers, capacitively coupled electro-thermaligniters, DC glow discharges, and DC cold cathode discharges.

[0021] Precursor materials are injected into the cathodic arc column byforced convection. Prior art teaches the injection velocity of theprecursor materials, with respect to the cathodic arc column, must becontrolled to enable the precursors to cross the arc column boundary toyield an efficient process. But precursors may also be aspirated intothe arc from the surrounding atmosphere in the absence of forcedconvection. The object of this invention is not limited by the method orefficiency by which precursors are introduced into the plasma—only thatthe precursors are introduced into the plasma and energy is transferredfrom the plasma to the precursors. The form of the precursor does notlimit the object of this invention; precursors are selected from a groupcomprising solids (powders, electrodes, etc.), liquids (atomized fluids,etc.) and gases or vapors.

[0022] The “active volume” is created through introduction of anoxidizing gas into the plasma, before the plasma is expanded into afield-free zone, either (1) in a region in close proximity to a zone ofcharge carrier generation, or (2) in a region of current conductionbetween field generating elements, including the surface of the fieldgeneration elements.

[0023] Energy is transferred from the plasma to a precursor material ormaterials and at least one of a stoichiometric-nanostructured materialand a vapor that may be condensed to form astoichiometric-nanostructured material is formed in the “active volume”.

[0024] Injecting at least one of a quench and dilution stream justbeyond the “active volume” enables additional control of the size of thestoichiometric-nanostructured material. The injection point beyond the“active volume” may vary from one mean free path of a plasma species(one collisional distance) to a larger distance deemed to be appropriateto quench the vapor and is generally determined by process equipmentconfiguration.

[0025] The stoichiometric-nanostructured material may be collected bymethods known to those familiar with the art.

EXAMPLE 1 Cerium Oxide—“Active Volume”

[0026] Two experiments utilizing nanostructured cerium oxide,synthesized with and without an “active volume” in the plasma, arepresented.

[0027] The plasma was generated using a free-burning electric arc. Theplasma gas was argon and the arc power was 62 kW.

[0028] The precursor material was particulate cerium oxide powder havingan average particle size greater than 2 microns and 99.95% pure. Theprecursor was fluidized with a feed gas to create a heterogeneousprecursor feed that was injected into cathodic arc column.

[0029] In Experiment 1 no “active volume” was created in the plasma. InExperiment 2 an “active volume” was created in the plasma by fluidizingthe precursor with an oxidizing gas—oxygen—to form a heterogeneousprecursor feed. In all other respects the two experiments were conductedunder identical conditions.

[0030] Experiment 1 uses an inert gas to fluidize the precursor and isrepresentative of prior teachings. In contrast, Experiment 2 creates an“active volume” in the plasma. Experiment 2 illustrates the teachings ofthis invention.

[0031] Both experiments yield nanostructured materials of similarparticle sizes (approximately 95 nm) but have very different surfacechemistry. The zeta potential for Experiment 1 and Experiment 2 materialare 2.6 mV and 43.5 mV, respectively. Experiment 1 material does notform stable aqueous dispersions without the aid of dispersants.

[0032] The stoichiometicly-nanostructured material produced inExperiment 2 has a very high zeta potential, exhibits high dispersionstability without additives, and is hydrolytically stable. Thestoichiometicly-nanostructure material produced in Experiment 2 hasgreat value in polishing applications.

EXAMPLE 2 Cerium Oxide—“Active Volume” with Quench and Dilution

[0033] Two experiments utilizing nanostructured cerium oxide,synthesized with and without an “active volume” in the plasma followedby quenching and dilution, are presented.

[0034] The plasma was generated using a free-burning electric arc. Theplasma gas was argon and the arc power was 62 kW.

[0035] The precursor material was particulate cerium oxide powder havingan average particle size greater than 2 microns and 99.95% pure. Theprecursor was fluidized with a feed gas to create a heterogeneousprecursor feed that was injected into cathodic arc column.

[0036] In Experiment 3 no “active volume” was created in the plasma. InExperiment 4 an “active volume” was created in the plasma by fluidizingthe precursor with an oxidizing gas—oxygen—to form a heterogeneousprecursor feed. A quench and dilution stream comprised of an oxidizinggas—oxygen—was injected just beyond the “active volume” in bothexperiments. In all other respects the two experiments were conductedunder identical conditions.

[0037] Experiment 3 is representative of prior teaching and uses aninert gas to fluidize the precursor and an oxidizing gas to quench anddilute the product. In contrast, Experiment 4 creates an “active volume”in the plasma and quenches and dilutes the product. Experiment 4illustrates the teachings of this invention.

[0038] Both experiments yield nanostructured materials of similarparticle sizes (approximately 30 nm) but have very different surfacechemistry. The zeta potential for Experiment 3 and Experiment 4 materialare 10.9 mV and 39.4 mV, respectively. Experiment 3 material does notform stable aqueous dispersions without the aid of dispersants. Thus,the injection of an oxidizing gas just beyond the “active volume”, as isshown in Experiment 3, is not sufficient to producestoichiometicly-nanostructure materials with high zeta-potentials,hydrolytic stability, and the ability to form stable aqueous dispersionswithout additives.

[0039] The stoichiometicly-nanostructure material produced in Experiment4 has a very high zeta potential, exhibits high dispersion stabilitywithout additives, and is hydrolytically stable. Thestoichiometicly-nanostructure material produced in Experiment 4 hasgreat value in polishing applications.

EXAMPLE 3 Extension of Process to Materials Other Than Cerium Oxide

[0040] The methods taught in this patent may be extended to materialsother than cerium oxide. For example, stable aqueous dispersions may beformed from the following materials listed with their zeta-potentials.Material Zeta-Potential Alumina 46.5 mV Antimony Tin Oxide −49.9 mVIndium Tin Oxide 37.9 mV

[0041] The preceding specific embodiments are illustrative of thepractice of the invention. It is to be understood, however, that otherexpedients known to those skilled in the art, or disclosed herein, maybe employed without departing from the spirit of the invention or thescope of the appended claims.

We claim:
 1. A process to prepare stoichiometric-nanostructuredmaterials comprising: generating a plasma; forming an “active volume”through introduction of an oxidizing gas into the plasma, before theplasma is expanded into a field-free zone, either (1) in a region inclose proximity to a zone of charge carrier generation, or (2) in aregion of current conduction between field generating elements,including the surface of the field generation elements; and transferringenergy from the plasma to a precursor material or materials and formingin the “active volume” at least one of stoichiometric-nanostructuredmaterials and a vapor that may be condensed to form astoichiometric-nanostructured material.
 2. The process of claim 1,wherein the step of generating comprises utilizing a radio-frequencyfield to generate the plasma.
 3. The process of claim 1, wherein thestep of generating comprises utilizing a microwave discharge to generatethe plasma.
 4. The process of claim 1, wherein the step of generatingcomprises utilizing a free-burning electric arc to generate the plasma.5. The process of claim 1, wherein the step of generating comprisesutilizing a transferred electric arc to generate the plasma.
 6. Theprocess of claim 1, wherein the step of generating comprises utilizing ahigh-intensity laser to generate the plasma.
 7. The process of claim 1,wherein the step of generating comprises utilizing a capacitivelycoupled electro-thermal igniter to generate the plasma.
 8. The processof claim 1, wherein the step of generating comprises utilizing a DC glowdischarge to generate the plasma.
 9. The process of claim 1, wherein thestep of generating comprises utilizing a DC cold cathode discharge togenerate the plasma.
 10. The process of claim 1, wherein the step offorming comprises selecting the oxidizing gas from one of a gascontaining oxygen atoms or a gas mixture containing oxygen atoms. 11.The process of claim 1, wherein the step of forming comprises selectingnon-oxygen components of the oxidizing gas from a group comprising He,Ne, Ar, Kr, Xe, N2, and H2, or mixtures thereof.
 12. The process ofclaim 1, wherein the step of forming comprises selecting N2O as theoxidizing gas.
 13. The process of claim 1, wherein the step of formingcomprises selecting O2 as the oxidizing gas.
 14. The process of claim 1,wherein the step of forming comprises selecting CO2 as the oxidizinggas.
 15. The process of claim 1, wherein the step of forming comprisesintroducing the oxidizing gas into a anodic column of a transferredelectric arc.
 16. The process of claim 1, wherein the step of formingcomprises introducing the oxidizing gas into a cathodic column of atransferred electric arc.
 17. The process of claim 1, wherein the stepof forming comprises introducing the oxidizing gas into a anodic columnof a free-burning electric arc.
 18. The process of claim 1, wherein thestep of forming comprises introducing the oxidizing gas into a cathodiccolumn of a free-burning electric arc.
 19. The process of claim 1,wherein the step of forming comprises introducing the oxidizing gas tothe plasma by natural convection.
 20. The process of claim 1, whereinthe step of forming comprises introducing the oxidizing gas to theplasma by forced convection.
 21. The process of claim 1, wherein thestep of forming comprises allowing the oxidizing gas to atomize a liquidnanoparticle precursor and introduce it into the “active volume”. 22.The process of claim 1, wherein the step of forming comprises allowingthe oxidizing gas to fluidize and transport a solid nanoparticleprecursor into the “active volume”.
 23. The process of claim 1, furthercomprising: Injecting at least one of a quench and dilution stream justbeyond the “active volume.” The injection point beyond the “activevolume” is from one mean free path of a plasma species to a largerdistance deemed to be appropriate to quench the vapor and is generallydetermined by process equipment configuration.
 24. The process of claim23, wherein the step of injecting comprises creating a nanoparticleaerosol of controlled particle size.
 25. Stoichiometric-nanostructuredmaterials produced through steps comprising: generating a plasma;forming an “active volume” through introduction of an oxidizing gas intothe plasma, before the plasma is expanded into a field free zone, in aregion in close proximity to either (1) a zone of charge carriergeneration, or (2) a region of current conduction between fieldgenerating elements, including the surface of the field generatingelectrodes; and transferring energy from the plasma to a precursormaterial or materials and forming in the “active volume” at least one ofstoichiometric-nanostructured materials and a vapor that may becondensed to form a stoichiometric-nanostructured material.
 26. Thestoichiometric-nanostructured materials of claim 25, wherein thestoichiometric-nanostructured materials are metal oxides.
 27. Thestoichiometric-nanostructured materials of claim 25, wherein thestoichiometric-nanostructured materials are substantially sphericalnanocrystalline metal oxides.
 28. The stoichiometric-nanostructuredmaterials of claim 26, wherein the metal oxides are selected from agroup comprising aluminum oxide, zinc oxide, iron oxide, cerium oxide,chromium oxide, antimony tin oxide, mixed rare earth oxides, and indiumtin oxide.
 29. The stoichiometric-nanostructured materials of claim 25,wherein the stoichiometric-nanostructured materials generally have asize distribution and range in mean diameter from about 1 nm to about900 nm.
 30. The stoichiometric-nanostructured materials of claim 29,wherein the stoichiometric-nanostructured materials generally have asize distribution and range in mean diameter from about 2 nm to about100 nm.
 31. The stoichiometric-nanostructured materials of claim 30,wherein the stoichiometric-nanostructured materials generally have asize distribution and range in mean diameter from about 5 nm to about 40nm.
 32. The stoichiometric-nanostructured materials of claim 25, whereinthe stoichiometric-nanostructured materials have a surface chemistryhaving a high aqueous dispersion stability.
 33. Thestoichiometric-nanostructured materials of claim 25, wherein thestoichiometric-nanostructured materials have a surface chemistry havinga low rate of hydrolysis.
 34. The stoichiometric-nanostructuredmaterials of claim 25, wherein the stoichiometric-nanostructuredmaterials have a surface chemistry with the absolute value of the zetapotential greater than 20 mV.
 35. The stoichiometric-nanostructuredmaterials of claim 34, wherein the stoichiometric-nanostructuredmaterials have a surface chemistry with the absolute value of the zetapotential greater than 30 mV.
 36. The stoichiometric-nanostructuredmaterials of claim 35, wherein the stoichiometric-nanostructuredmaterials have a surface chemistry with the absolute value of the zetapotential greater than 35 mV.